1. Field of the Invention
This invention relates to an improved process for producing carbon fibers from pitch which has been transformed, in part, to a liquid crystal or so-called "mesophase" state. More particularly, this invention relates to an improved process for producing carbon fibers from pitch of this type wherein the mesophase content is produced in substantially shorter periods of time than heretofore possible, at a given temperature, by subjecting the pitch to reduced pressure during formation of the mesophase.
2. Description of the Prior Art
As a result of the rapidly expanding growth of the aircraft, space and missile industries in recent years, a need was created for materials exhibiting a unique and extraordinary combination of physical properties. Thus, materials characterized by high strength and stiffness, and at the same time of light weight, were required for use in such applications as the fabrication of aircraft structures, re-entry vehicles, and space vehicles, as well as in the preparation of marine deep-submergence pressure vessels and like structures. Existing technology was incapable of supplying such materials and the search to satisfy this need centered about the fabrication of composite articles.
One of the most promising materials suggested for use in composite form was high strength, high modulus carbon textiles, which were introduced into the market place at the very time this rapid growth in the aircraft, space and missile industries was occurring. Such textiles have been incorporated in both plastic and metal matrices to produce composites having extraordinary high-strength- and high-modulus-to-weight ratios and other exceptional properties. However, the high cost of producing the high-strength, high-modulus carbon textiles employed in such composites has been a major deterrent to their widespread use, in spite of the remarkable properties exhibited by such composites.
One recently proposed method of providing high-modulus, high-strength carbon fibers at low cost is described in copending application Ser. No. 338,147, now U.S. Pat. No. 4,005,183, entitled "High Modulus, High Strength Carbon Fibers Produced From Mesophase Pitch." Such method comprises first spinning a carbonaceous fiber from a carbonaceous pitch which has been transformed, in part, to a liquid crystal or so-called "mesophase" state, then thermosetting the fiber so produced by heating the fiber in an oxygen-containing atmosphere for a time sufficient to render it infusible, and finally carbonizing the thermoset fiber by heating in an inert atmosphere to a temperature sufficiently elevated to remove hydrogen and other volatiles and produce a substantially all-carbon fiber. The carbon fibers produced in this manner have a highly oriented structure characterized by the presence of carbon crystallites preferentially aligned parallel to the fiber axis, and are graphitizable materials which when heated to graphitizing temperatures develop the three-dimensional order characteristic of polycrystalline graphite and graphitic-like properties associated therewith, such as high density and low electrical resistivity. At all stages of their development from the as-drawn condition to the graphitized state, the fibers are characterized by the presence of large oriented elongated graphitizable domains preferentially aligned parallel to the fiber axis.
When natural or synthetic pitches having an aromatic base are heated under quiescent conditions at a temperature of about 350.degree. C.- 500.degree. C., either at constant temperature or with gradually increasing temperature, small insoluble liquid spheres begin to appear in the pitch and gradually increase in size as heating is continued. When examined by electron diffraction and polarized light techniques, these spheres are shown to consist of layers of oriented molecules aligned in the same direction. As these spheres continue to grow in size as heating is continued, they come in contact with one another and gradually coalesce with each other to produce larger masses of aligned layers. As coalescence continues, domains of aligned molecules much larger than those of the original spheres are formed. These domains come together to form a bulk mesophase wherein the transition from one oriented domain to another sometimes occurs smoothly and continuously through gradually curving lamellae and sometimes more sharply curving lamellae. The differences in orientation between the domains create a complex array of polarized light extinction contours in the bulk mesophase corresponding to various types of linear discontinuity in molecular alignment. The ultimate size of the oriented domains produced is dependent upon the viscosity, and the rate of increase of the viscosity, of the mesophase from which they are formed, which, in turn are dependent upon the particular pitch and the heating rate. In certain pitches, domains having sizes in excess of two hundred microns up to in excess of one thousand microns are produced. In other pitches, the viscosity of the mesophase is such that only limited coalescense and structural rearrangement of layers occur, so that the ultimate domain size does not exceed one hundred microns.
The highly oriented, optically anisotropic, insoluble material produced by treating pitches in this manner has been given the term "mesophase," and pitches containing such material are known as "mesophase pitches." Such pitches, when heated above their softening points, are mixtures of two essentially immiscible liquids, one the optically anisotropic, oriented mesophase portion, and the other the isotropic non-mesophase portion. The term "mesophase" is derived from the Greek "mesos" or "intermediate" and indicates the pseudo-crystalline nature of this highly-oriented, optically anisotropic material.
Carbonaceous pitches having a mesophase content of from about 40 percent by weight to about 90 percent by weight are suitable for spinning into fibers which can subsequently be converted by heat treatment into carbon fibers having a high Young's modulus of elasticity and high tensile strength. In order to obtain the desired fibers from such pitch, however, it is not only necessary that such amount of mesophase be present, but also that it form, under quiescent conditions, a homogeneous bulk mesophase having large coalesced domains, i.e., domains of aligned molecules in excess of two hundred microns up to in excess of one thousand microns in size. Pitches which form stringy bulk mesophase under quiescent conditions, having small oriented domains, rather than large coalesced domains, are unsuitable. Such pitches form mesophase having a high viscosity which undergoes only limited coalescense, insufficient to produce large coalesced domains having sizes in excess of two hundred microns. Instead, small oriented domains of mesophase agglomerate to produce clumps or stringy masses wherein the ultimate domain size does not exceed one hundred microns. Certain pitches which polymerize very rapidly are of this type. Likewise, pitches which do not form a homogeneous bulk mesophase are unsuitable. The latter phenomenon is caused by the presence of infusible solids (which are either present in the original pitch or which develop on heating) which are enveloped by the coalescing mesophase and serve to interrupt the homogeneity and uniformity of the coalesced domains, and the boundaries between them.
Another requirement is that the pitch be nonthixotropic under the conditions employed in the spinning of the pitch into fibers, i.e., it must exhibit a Newtonian or plastic flow behavior so that the flow is uniform and well behaved. When such pitches are heated to a temperature where they exhibit a viscosity of from about 10 poises to about 200 poises, uniform fibers may be readily spun therefrom. Pitches, on the other hand, which do not exhibit Newtonian or plastic flow behavior at the temperature of spinning, do not permit uniform fibers to be spun therefrom which can be converted by further heat treatment into carbon fibers having a high Young's modulus of elasticity and high tensile strength.
Carbonaceous pitches having a mesophase content of from about 40 percent by weight to about 90 percent by weight can be produced in accordance with known techniques, as disclosed in aforementioned copending application Ser. No. 338,147, now U.S. Pat. No. 4,005,183, by heating a carbonaceous pitch in an inert atmosphere at a temperature above about 350.degree. C. for a time sufficient to produce the desired quantity of mesophase. By an inert atmosphere is meant an atmosphere which does not react with the pitch under the heating conditions employed, such as nitrogen, argon, xenon, helium, and the like. The heating period required to produce the desired mesophase content varies with the particular pitch and temperature employed, with longer heating periods required at lower temperatures than at higher temperatures. At 350.degree. C., the minimum temperature generally required to produce mesophase, at least one week of heating is usually necessary to produce a mesophase content of about 40 percent. At temperatures of from about 400.degree. C. to 450.degree. C., conversion to mesophase proceeds more rapidly, and a 50 percent mesophase content can usually be produced at such temperatures within about 1-40 hours. Such temperatures are generally employed for this reason. Temperatures above about 500.degree. C. are undesirable, and heating at this temperature should not be employed for more than about 5 minutes to avoid conversion of the pitch to coke.
Although the time required to produce a mesophase pitch having a given mesophase content is reduced as the temperature of preparation rises, it has been found that heating at elevated temperatures adversely affects the rheological properties of the pitch by altering the molecular weight distribution of both the mesophase and non-mesophase portions of the pitch. Thus, heating at elevated temperatures tends to increase the amount of high molecular weight molecules in the mesophase portion of the pitch. At the same time, heating at such temperatures also results in an increased amount of low molecular weight molecules in the non-mesophase portion of the pitch. As a result, mesophase pitches of a given mesophase content prepared at elevated temperatures in relatively short periods of time have been found to have a higher average molecular weight in the mesophase portion of the pitch and a lower average molecular weight in the non-mesophase portion of the pitch, than mesophase pitches of like mesophase content prepared at more moderate temperatures over more extended periods. This wider molecular weight distribution has been found to have an adverse effect on the rheology and spinnability of the pitch, evidently because of a low degree of compatibility between the very high molecular weight fraction of the mesophase portion of the pitch and the very low molecular weight fraction of the non-mesophase portion of the pitch. The very high molecular weight material in the mesophase portion of the pitch can only be adequately plasticized at very high temperatures where the tendency of the very low molecular weight molecules in the non-mesophase portion of the pitch to volatilize is greatly increased. As a result, when such pitches are heated to a temperature where they have a viscosity suitable for spinning and attempts are made to produce fibers therefrom, excessive expulsion of volatiles occurs which greatly interferes with the processability of the pitch into fibers of small and uniform diameter. For these reasons, means have been sought for shortening the time required to produce mesophase pitch at relatively moderate temperatures of preparation where more favorable rheological properties are imparted to the pitch.